In many medical procedures, devices, such as probes, endoscopes, catheters, stents and tags/markers are inserted into a patient's body. Such devices are used for a large variety of procedures including irreversible surgical actions, such as ablation and taking of tissue samples. Therefore, it is necessary to have accurate information on the position and orientation of the probe within the patient's body.
Electromagnetic position determining systems provide a convenient method of receiving accurate information on the position and orientation of intra-body objects, and allow accurate tracking of these objects. Such systems are described for example in U.S. Pat. Nos. 5,558,091, 5,391,199 and 5,443,489, and in International Patent Publications WO94/04938 and WO96/05768, whose disclosures are incorporated herein by reference. These systems determine the coordinates of a device using one or more field sensors, such as a Hall effect device, coils or other antennas carried on the device. The field sensors are transducers used as position sensors and are typically located at or adjacent the distal end of the device, and/or along the length of the device. Therefore, the transducers are preferably made as small as possible so as to fit into the device without interfering with the device's maneuverability or increasing its size unduly.
U.S. Pat. No. 5,558,091 describes a Hall effect sensor assembly of a cube shape which includes three mutually orthogonal, thin galvanomagnetic films. This sensor assembly is preferably of dimensions about 3×0.75×0.75 mm. The U.S. Pat. No. 5,558,091 Patent further describes another Hall effect sensor assembly which includes three field sensing elements in the form of semiconductor chips. Each chip includes one or more elongated bars of a magnetoresistive material. Each such chip is sensitive to magnetic field components in the direction of the bar. This assembly preferably has a diameter of 0.8 mm or less. However, such chips suffer from nonlinearities, saturation effects, hysteresis and temperature drifts.
Therefore, most magnetic position determining systems use sensors formed of miniature coils that include a large number of turns of an electrically conducting wire. Such coils are described, for example, in PCT publications PCT/GB93/01736, WO94/04938 and WO96/05768, in the above mentioned U.S. Pat. No. 5,391,199, and in PCT publication PCT/IL97/00009, which is assigned to the assignee of the present application, all of which are incorporated herein by reference. The performance of a sensor coil is dependent on its inductance, which is a function of the number of turns of the coil times the cross sectional area of the coil. Therefore, in planning and designing a miniature coil for use within a surgical device, for example, it is generally necessary to make a compromise between performance and the size of the coil. Such coils are typically used in position sensors having three mutually orthogonal sensor coils and typically have minimum dimensions of 0.6×0.6×0.6 mm and more generally of 0.8×0.8×0.8 mm. It has always been believed that smaller coils of the same type would not provide acceptable performance and are also hard to manufacture. Additionally, given these fixed size limitations, no sensor coils have been developed that have an outer diameter less than 0.6 mm.
Moreover, for these types of position sensors, it is common for the sensor coil to include a core. For those position sensors (sensor coil) that utilize a core, it is known that the material for the core can consist of two acceptable materials. The first material is ferrite and has been used with success as a core material for medical devices having a sensor coil with a core.
The later core material developed that has also proved to be effective as core material for a sensor coil in a medical device is carbonyl iron. However, for both types of core materials, the sensor coils utilizing such core material would be generally limited to the outer diameter minimum dimension requirements described above.
In order to determine both translational and rotational coordinates, some position determining systems, such as the system described in the above-mentioned PCT publication WO96/05768, use three sensor coils, having respective axes that are mutually linearly independent, preferably mutually orthogonal. Preferably, these three coils are packaged together to form a sensor assembly, which is used to provide six-dimensional position and orientation coordinate readings. The use of an assembly which has the three coils within one package allows easy insertion and/or attachment of the coils to devices such as catheters. Also, the assembly provides exact positioning of the coils relative to each other, thus simplifying the calibration of position determining systems using the coils. Generally, the coils are enclosed in a cylindrical-shaped case, which protects the coils from the surroundings.
In the system of the '768 publication, this assembly typically has a length of about 6 mm and a diameter of about 1.3 mm. Because the axes of the coils need to be generally mutually orthogonal in order to achieve accurate position sensing in all six dimensions, it is not possible to make the diameter of the assembly much smaller.
Although this coil assembly fits into most medical devices, in some cases coils of equivalent performance and smaller width are desired. For example, U.S. Pat. No. 6,203,493, which is assigned to the assignee of the present invention and is incorporated herein by reference, describes a method of enhancing the accuracy of position determination of an endoscope that includes miniature position sensing coils, by distancing the coils from metallic apparatus within the endoscope. If the coil assembly can be made with a smaller width, it is then possible to increase the separation between the miniature coils and the metallic apparatus, and thus achieve better accuracy from the position determining system.
Coils made by photolithography or VLSI procedures are known as disclosed in U.S. Pat. No. 6,201,387 B1, which disclosure is incorporated herein by reference, in which these coils are referred to as photolithographic coils. Photolithographic coils are generally made in the form of a spiral conductor printed on a substrate of plastic, ceramic or semiconductor material. Such coils conventionally comprise up to four overlapping spiral layers, using currently available fabrication techniques.
Photolithographic coils or antennas are also commonly used in contactless smart cards, as are known in the art. These cards inductively communicate with and receive power from a reader circuit through a photolithographic coil or antenna embedded in the card. Because smart cards are limited in thickness to less than 0.8 mm, they generally include only a single coil, whose axis is necessarily perpendicular to the plane of the card. To communicate with the reader, the smart card must be properly oriented, so that the coil axis is aligned with a magnetic field generated by the reader, in order to achieve proper coupling.
Reducing the width or outer diameter of the coil assembly would allow position determining systems to be used with narrower devices, which generally have superior maneuverability and ease of access to remote locations. Alternatively, reducing the width or outer diameter of the coil assembly would allow the assembly to occupy a smaller portion of the cross-sectional area of the device, leaving more space for functional apparatus and/or working channels along the devices.
To date, there have been no position sensors or sensor coils having outer diameters that are smaller in size than the sensors described above and are capable of achieving performance measures such as maintaining a high degree of accuracy at high temperatures.